The Programmer's Guide to TRSDOS Version 6 - Tim Mann's Home ...

A high level language permits you the opportunity of indicating your access level in the
language syntax. For example, BASIC requires you to specify whether a sequentially
accessed file is to be INPUT or OUTPUT corresponding to READ or WRITE. The operating
system has no facility for identifying the maximum level of access desired for any
particular opening of the file except through the passwords and access protection level.
6.2.3 Opening Files
Files opened with UPDATE or greater access are indicated as open in their directory entry
record by the setting of a "file open bit". Any subsequent open attempt will result in a
force to READ access protection and return the appropriate "File already open" error
code. This is designed primarily for the use of shared access multiplexed disk drives
where files are shared among a number of users. This arrangement will restrict the
altering (but not reading) of file data to only one user at a time. It is therefore
important for applications to CLOSE files as soon as the application is finished with the
file access. It is also important for applications to trap the "File already open" error
and take appropriate action. Realize that files protected to READ only, may be opened by
multiple users and still be opened for updating by the maintainer providing the proper
OWNER password is provided. The importance of maintaining proper levels of file
protection through the use of passwords and protected access levels should not be taken
lightly.
For the convenience of applications that access files only for reading, a facility for
forcing the file access to READ only when a file is opened has been provided in the DOS.
This facility will inhibit the "file open bit" and set the File Control Block access
permission to READ (providing that the access permission level granted according to the
password entered was READ or greater). Under this linkage, it is not necessary to close
the file when you are finished accessing it as no directory updating will be done. Of
course if you want the system to recover the filespec and place it into the FCB , you will
have to close the file. Check the discussion covering the FORCE-to-READ flag (bit-0 of
the SFLAG$) in the @FLAGS SuperVisor Call. Note that once the FORCE-to-READ flag has been
set, the next @OPEN or @INIT SuperVisor Call request will automatically reset the bit
after satisfying the request.
When a file is opened, the system needs to be told where the disk file I/O buffer is
located. This buffer is used to transfer a full sector of data to and from the disk. The
system also needs to be told what Logical Record Length (LRL) is to be used while the
file is open. If the LRL at open time differs from the LRL of the file as noted in the
directory, the OPEN routine will return an "LRL open fault" error code BUT THE FILE WILL
STILL BE PROPERLY OPENED ACCORDING TO THE LRL PASSED IN THE OPEN REQUEST. The error code
is your indication that a different LRL is being used. If the LRL is 256, then the system
does not block and deblock the data records and will expect that all data to I/O will be
using the disk file I/O buffer. If the LRL is in the range , then the disk file
I/O buffer is used only for transferring full sectors to and from the disk. Say, for
example, a file has 200-byte records, the second record of the file is partially
contained in the first sector and partially contained in the second sector. The file is
said to SPAN two sectors. This requires a separate buffer to hold the record data while
the system uses the disk file I/O buffer for the transfer of the sector. The program then
will specify a USER RECORD buffer (UREC) that will be used by the system to transfer the
data records to and from the disk file I/O buffer on each I/O request. Thus, whenever a
file record spans two sectors, the system will have the necessary buffering regions to
fully block and deblock the record. Note that the arrangement of separate disk file I/O
buffers for each file provides greater flexibility for accessing multiple files
coincidentally.
To illustrate the linkage necessary to open an existing file, we will be referencing an
80-byte record length file with the specification, BULKLOAD/DAT:2. The file has an OWNER
password, blank USER password with protection level of WRITE. The filespec has been
placed into the File Control Block as shown in figure 6-2. Note that the filespec is left
justified and is terminated with an ETX (X'03') character. The ETX is automatically
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